1. Field of the Invention
The present invention relates a metal oxide electrode coated with a porous metal film, a metal oxide film or a carbon film, its fabrication method and a lithium-ion secondary battery using it.
2. Description of the Background Art
A cathode for lithium-ion secondary battery comprises a metal oxide active material for releasing and inserting lithium, a conductive material added to give conductivity, and a coupling material for fixing the metal oxide active material and the conductive material to an aluminum collecting body.
Since an electric conductivity varies depending on a particle size of the active material and a filling density, content and type of the conductive material and the coupling material, it is necessary to consider from a comprehensive view.
Among them, the metal oxide used as a cathode active material for a lithium-ion secondary battery has a bad electric conductivity due to its material characteristic, the role of the conductive material is crucial in view of smoothly supply electrons between packed active materials and from the collecting body to the surface of the active material (D. Linden, Handbook of Batteries, McGRAW-HILL INC., New York (1995)).
As the material oxide cathode, LiCoO2, LiMn2O4, LiNiO2, V6O13, V2O5 and the like are used. These have a low electric conductivity, so that in order to be used as an electrode, a conductive material such as acetylene black, carbon black or graphite should be used and a further amount of conductive material should be added.
With the increase of the addition amount of the conductive material, the amount of the coupling material is accordingly increased, and thus, an optimization for the amount of the added conductive material and the coupling material is needed, according to which a difference in a battery performance occurs.
For example, if the combination of the active material, the conductive material and the coupling material is not uniform, performance difference between electrodes resulted in a uniformity of the electrode performance, causing a problem of a reliability of the battery.
The coupling material serves to prevent desorption of the active material and heighten a bonding force between the active materials, but if added more than required, a battery performance is degraded due to reduction of the electrode active material and increase of an internal resistance.
Thus, there is a restriction to heighten the battery performance only with increase of the amount of the conductive material.
A conventional lithium-ion secondary battery uses a compound which includes lithium such as LiCoO2 or LiMn2O4 as a cathode. Thus, such a battery is fabricated without having lithium in carbon electrode used as an anode.
In case of the use of carbon electrode in a lithium-ion secondary battery, in the initial charging of the battery, a passive film is formed on the surface of the carbon electrode. The passive film interrupts so that an organic solvent is not inserted between the carbon grid layer, thereby restraining a dissolution reaction of the organic solvent, and thus, the passive film improves a stabilization of the carbon structure and the reversibility of the carbon electrode, making it possible to use the carbon electrode as a lithium-ion secondary anode.
However, such formation of a passive film is an irreversible reaction, bringing about a contrary effect that it causes the consumption of the lithium ions and thereby a reduction in the capacity of the battery.
In addition, a charging and discharging efficiency of the carbon electrode and the cathode is not completely 100%, as the cycle proceeds, lithium ion is consumed to cause a reduction in the electrode capacity, resulting in that the life cycle of the battery is degraded.
Therefore, an object of the present invention is to provide a porous metal oxide film or a porous carbon film, its fabrication method and a lithium-ion secondary battery using it, that are capable of increasing a capacity by compensating a capacity degradation due to an irreversible capacity in the carbon electrode generated at the initial stage of charging of the lithium-ion secondary battery, and capable of improving a life cycle by compensating for the amount of lithium consumed due to a low efficiency in charging and discharging.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a metal oxide electrode for a lithium-ion secondary battery, coated with a porous metal layer, a porous metal oxide layer or a porous carbon layer thin film having a thickness of a few xc3x85xcx9ca few xcexcm.
To achieve the above object, there is further provided a method for fabricating a metal oxide electrode coated with a porous metal, metal oxide or carbon, including the steps of: positioning a metal oxide electrode roll within a vacuum chamber; winding the metal oxide electrode off the roll at a certain speed, winding the metal oxide electrode on a different roll and coating porous metal or carbon with a thickness of a few xc3x85xcx9ca few xcexcm on the metal oxide electrode between the two rolls from a metal or carbon evaporation source; and stabilizing the metal oxide electrode in a vacuum state for a predetermined time period at a predetermined temperature.
To achieve the above object, there is further provided a lithium-ion secondary battery including a cathode fabricated with an active material such as LiCoO2, LiMn2O4, LiNiO2, V6O13, V2O5 and the like, and coated with a porous metal or a carbon thin film with a thickness of a few xc3x85xcx9ca few xcexcm, and an anode fabricated with an active material such as graphite, coke or hard carbon.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.